The contamination of surface and near-surface soils has become a matter of great concern in many locations in the United States and other countries. Increasingly, the public has become aware of spills and landfills of toxic and carcinogenic or radioactive materials in the soil. If left in place, many of these contaminants will find their way into aquifers, air, or into the food supply, and could become public health hazards.
There are many proposed methods for removal of surface contaminants, such as excavation followed by incineration, in situ vitrification, biological treatment, chemical additives for deactivation, radiofrequency heating, etc. Although successful in some applications, these methods can be very expensive (hundreds of dollars per ton) and are not practical if many tons of soil must be treated.
Hazardous waste materials have been improperly deposited in thousands of sites all over the United States and, indeed, all over the world. Uncontrolled landfills have been used as convenient, but inadequate, disposal sites for industrially generated wastes while other sites have been contaminated by accidental spills of hazardous materials. There are many sites where hazardous materials found at these sites are stable, do not undergo environmental degradation at reasonably fast rates, have high boiling points, are considered toxic at very low concentration levels, and bio-accumulate in various species of the food chain at concentrations higher than that found in the environment.
Complete reclamation or isolation of such sites is preferred but the cost associated with site remediation by available methods has been considered prohibitive. The treatment of contaminated soil from such sites in an incinerator has not been a practical solution for several reasons including the high cost of excavation and incineration, shortage of incineration capacity, inadequate methods and capacity for ash disposal from the incinerators, and the hazards and risks associated with site disturbance and transportation. The treatment of uncontrolled landfills and spills would benefit from an in situ process that eliminates or alleviates these disadvantages and risks.
In situ heating of earth formations by high-frequency displacement currents (dielectric heating) is well known, particularly in the production of petroleum products such as shale oil. Alternatively, heating by conduction currents at relatively low frequencies is also possible, but such heating is limited to earth that remains conductive, generally requiring the presence of water and, hence, operating at relatively low temperatures below the boiling point of water or requiring maintenance of pressure. Conduction heating at very high temperatures for the immobilization of radioactive components in soil is shown in Brouns et al, U.S. Pat. No. 4,376,598, where conductive material was added to the soil to assure conduction, and the soil was heated to vitrification at temperatures as high as 1500.degree. C., whereat radioactive contaminants are fused with the silicates in the soil to form a glass or similar product which, upon cooling, forms a stable mass.
In situ heating of earth formations with RF energy for hydrocarbon production is shown in Bridges et al, U.S. Pat. No. RE 30,738 and Kasevich et al, U.S. Pat. No. 4,140,179. The former discloses the use of RF energy from a "tri-plate" line buried in the earth to heat a block of earth formations uniformly by displacement currents, leading to dielectric heating. The latter discloses radiating RF energy into the earth. In U.S. Pat. No. 4,670,634 a portion of the earth near the surface is decontaminated by selective heating with RF energy from a transmission line array to which the RF energy is bound. That is, there is substantially no radiation from the bound-wave fringing-field transmission line excitor.
Assignee's co-pending application Ser. No. 444,574 filed Nov. 20, 1989, now U.S. Pat. No. 4,973,811 utilizes a different coupling scheme, called "eddy current" "inductive" coupling which makes use of the magnetic field established rather than the electric fields as disclosed in the prior art RF systems above-described. A current generator is used to drive a transmission line which is shorted at the opposite ends. This method is much better suited to heating highly conductive soils than the fringe field method.
U.S. Pat. No. 4,842,448 issued to Robert M. Koerner et al on Jun. 27, 1989 discloses a method and apparatus for in situ removal of contaminants from soil comprising a barrier having a permeable inner layer and an impermeable outer layer overlying the contaminated soil and a vacuum system for reducing pressure under the barrier and withdrawing contaminants from the contaminated soil.
Assignee's co-pending application Ser. No. 427,418 filed Oct. 27, 1989, now U.S. Pat. No. 4,984,594 discloses an in situ method for remediation and decontamination of surface and near-surface soils by evacuating the soil under a flexible sheet, which is impermeable to gases, and heating the soil surface with a relatively flat electric surface heater, which is permeable to gases.
In assignee's co-pending application Ser. No. 833,569 filed Feb. 7, 1992, which is a continuation of Ser. No. 427,427 filed Oct. 27, 1989, now abandoned, an in situ method is disclosed for remediation and decontamination of surface and near-surface soils by electrically heating the soil through electrodes operated at power line frequencies of about 60 Hz. The electrodes are implanted substantially vertically in the soil in a line pattern which allows substantially uniform electrical heating in the region between rows of electrodes. The depth of electrode insertion is substantially equal to the depth of the contamination, but could be deeper or shallower. The process is particularly applicable to soils contaminated at depths of up to about 30 meters. The electrodes are hollow and perforated below the surface to allow application of a vacuum to the soil through the electrodes. The electrodes are also equipped with provision for electrical connection and vacuum line connection, and also with the capability to be sealed to a barrier that is impermeable to gases, such as a flexible sheet.
U.S. Pat. No. 4,435,292 discloses a portable system which can be installed at an area where a contaminating spill has occurred. After the area of the contamination has been determined, perforated pipes are inserted into the ground. Some of the wells are pressurized and others are evacuated simultaneously so as to increase the transfer of a flushing fluid through the soil thereby accelerating the decontamination process and preventing migration of the contaminant into other areas. Since the system is a closed system, the contaminated liquid taken off in the evacuation side of the circulating system is bypassed in whole or in part to a portable processing plant wherein the contaminants are removed.
And finally, in Assignee's co-pending application Ser. No. 559,771 filed Jul. 30, 1990, now U.S. Pat. No. 5,076,727 moist warm air from a vapor treatment system is injected into wells which are screened (perforated) only at the contaminated depth forcing vapor flow only through the contaminated region. Between the injection wells is an extraction well which is also screened only at the contaminated depth. A vacuum is drawn on the extraction well through the contaminated soil, thereby entraining some of the contaminants. The contaminated, flushing vapor is then treated and recycled. A microwave/radio frequency (MW/RF) heating system heats the earth's surface and the contaminated soil, thereby enhancing volatilization of the contaminants and their removal via the vapor flushing system. By screening the wells only through the contaminated zone and maintaining the contaminated soil zone in a moist state, the entire energy of the system is focussed on the contaminated region.
Most of the existing methods for decontamination are cost effective only for the near surface soil, of the order of several feet. For deeper contamination, particularly in localized regions, the costs of soil removal rise dramatically.
What is desired, therefore, is a method for effectively removing contamination which avoids the excavation of large quantities of soil; which can be utilized at considerable depths up to many tens or hundreds of feet; which can remove contaminants with boiling points substantially greater than water (up to hundreds of degrees C.); which is generally applicable to a wide range of contaminants (even those with low vapor pressure at room temperature); which can reduce the level of contamination down to extremely low levels of the order of parts per billion; and which is rapid and inexpensive to operate in a wide variety of soils and surface conditions. These and other advantages of this invention are disclosed below.